This invention relates to a video signal processing circuit for use in color television for processing a video signal according to motion of a picture. More particularly, this invention relates to a Y/C separation circuit separating a video signal into a luminance signal and a color signal, in a television signal processing circuit in which a scanning line is interpolated between scanning lines of a television signal of interlace system to double the number of scanning lines to be used in a line sequential color television system, a video signal processing circuit of motion adaptive type in which Y/C separation or generation of an interpolation scanning signal can be appropriately achieved depending on whether a picture is a still picture or a moving picture.
The operation of a Y/C separation circuit is such that, in the case of a still picture, a luminance signal and a color signal are separated by arithmetic operation with a video signal applied in the preceding frame, while, in the case of a moving picture, the luminance signal and the color signal are separated by arithmetic operation with the video signal of the same field. When the Y/C separation is made according to the interframe arithmetic operaiton in the case of a moving picture, the absence of interframe correlation results in occurrence of interference. Therefore, on the basis of a detection output signal of a motion detecting circuit discriminating whether a picture is a still picture or a moving picture on the basis of difference signal between a present signal and a signal earlier by one frame or more frames, the mixing ratio between the Y/C separation output obtained by the arithmetic operation between the frames and the Y/C separation output obtained by the arithmetic operation in the same field is preferably controlled to produce a luminance signal and a color signal of high picture quality. Such a method is already known.
When a picture to be displayed is a still picture in the case of scanning line interpolation, a line signal representative of a scanning line appeared in a preceding field earlier by one field than the present field can be directly interpolated between scanning lines. However, when a picture to be displayed is a moving picture, interpolation of a scanning line on the basis of a line signal appeared in the preceding field is not satisfactory in that continuous motion of part of the picture within 1/60 seconds results in undesirable degradation of the quality of that part of the picture. Therefore, in the case of a moving picture, it is a common practice that an average of scanning lines upper and lower than the specific scanning line in the same field is based to determine the interpolating scanning line. In this connection, it is known that, on the basis of a detection output signal of a motion detecting circuit identifying whether a picture is a moving picture or a still picture by detecting the difference between a line signal appeared in a field earlier by one frame or more frames and a line signal appearing in the present field, the mixing ratio between the line signal appeared in the preceding field and an average signal of a line signal appeared at time earlier by one scanning line and that of the present scanning line is controlled to improve the quality of the picture.
A technique for displaying a picture of high quality by processing a color television signal is disclosed in, for example, No. JP-A-58-177078. The object of the above publication is to lessen cross color interference, dot crawl interference and the like attributable to frequency multiplexing superposition of a color signal on a luminance signal and to prevent interference such as line flickering occurring at edges of lateral fringes due to the interlace scanning and also to prevent lowering of the resolution. In order to improve the quality of a still picture, a Y/C separation circuit (or a frame comb circuit) is required, which utilizes the fact that the subcarrier is frequency interleaved and in which a Y signal (the luminance signal) and a C signal (the color signal) are separated from each other on the basis of the difference signal between frames. In addition to the Y/C circuit, it is necessary to interpolate a scanning line using the one appeared in the preceding field. However, in the case of a moving picture, there is not always a coincidence between frame signals. In such a case, the Y/C separation and the interfield scanning line interpolation will rather degrade the picture quality. Therefore, a signal processing circuit of motion adaptive type is utilized in which, when a picture is detected to be a still picture, Y/C separation by Y/C separation circuit and interfield scanning line interpolation are carried out as described above, but, when the picture is detected as a moving picture, signal processing is always carried out in the same field.
A composite color television signal is an interlace signal, and its subcarrier is frequency interleaved. Therefore, the arrangement of scanning lines of the television signal is as shown in FIG. 2 in which the horizontal axis represents time, and the vertical axis represents scanning lines arranged in a direction vertical to the screen, that is, vertical to the drawing sheet. In FIG. 2, the arrow indicates the phase of the subcarrier. It will be seen from FIG. 2 that, when motion of a picture between frames is detected on defferent scanning lines, such motion is always detected at a vertical edge portion, and, therefore, motion is commonly detected on the basis of difference signal between a present signal and a signal earlier by one frame or more frames.
The prior art circuit described above can make optimum Y/C separation and scanning line interpolation thereby improving the picture quality when motion of a picture is accurately detected. However, when motion of a picture is very quick, motion detection according to the interframe difference signal may make errors in detection of the motion of the picture, and various interference as described above occur when the Y/C separation and the scanning line interpolation are carried out on the basis of the erroneous motion information.
Consider now, for example, a scanning line interpolation circuit of motion adaptive type. FIG. 3 shows schematically that a black body having a size corresponding to three lines moves from a lower position toward an upper position on a screen. In FIG. 3, the horizontal axis represents time, and the vertical axis represents the vertical direction. Symbols S.sub.0 designates a present scanning line signal, S.sub.1 designates a scanning line signal appeared at time earlier by 1H than the present time, and so on. It will be apparent from FIG. 3 that motion information for providing an interpolation signal of an M-th field illustrated by a square mark is produced on the basis of a line signal S.sub.525 of an (M-1)th field and a line signal S.sub.0 of an (M+1)th field. However, since the picture in the form of the black rectangle moves quickly in the vertical direction in one field period as shown in FIG. 3, there is not any substantial difference between the line signal S.sub.525 of the (M-1)th field and the line signal S.sub.0 of the (M+1)th field, and the picture appearing in the M-th field is decided as a still picture. Therefore, the interpolation signal of the M-th field is produced as a bright signal from the bright signals of the (M-1)th and (M+1)th fields, and a problem arises such that the square mark which should be a black picture element is interpolated as a bright part.